1. Field of the Invention
The invention relates to light emitting signs and light emitting display surfaces for generating fixed images, graphics, photographic images and characters of a desired color of light. In particular the invention concerns light emitting signs which utilize a semiconductor light emitting diode (LED) and a phosphor (photo luminescent) material to generate a desired color of emitted light. Moreover the invention relates to generating colored light over large surface areas.
2. Description of the Related Art
Light emitting signs/displays, sometimes termed illuminated signs or displays, are used in many applications including: name signs for business premises using fixed graphics and characters, fixed image signs for advertising, emergency signs such as exit signs, traffic signals, road signs for example speed limit, stop, give way (yield) signs, direction indicator signs to name but a few.
A common way to make light emitting signs is in the form of a backlit sign or display which uses a “light box” containing one or more white light source such as for example fluorescent tubes, neon lights or incandescent bulbs. A front panel of the display comprises a transparent color filter, often a colored transparent acrylic sheet, which selectively filters the white light to provide the desired color light emission, graphic or image. Often, the light box is custom fabricated from sheet metal as a rectangular box or as a box in the shape of a required letter/character/symbol (channel letter) and such construction in conjunction with the white light source can account for a significant proportion of the total cost of the sign. The color pigments, dyes or colorants, used in these systems are transparent color filters which absorb the unwanted color light. This method is used for most light emitting signs and fixed displays as well as light emitting transparencies and many colored lights. A disadvantage of such signs is that a color filter has to be fabricated for every color required which increases the cost. In practice to minimize cost, the number of colors is limited to twenty or so. In addition, while such signs give a good performance at night they give poor color performance in daylight conditions due to their mode of operation which relies on the transmission rather than reflection of light and such signs can appear “washed out”. Moreover, increasing the brightness of the signs leads to a bleeding through of the white backlight which leads to a shift in color saturation, e.g. deep red is washed out and appears whitish (pink) red. This effect is due to the “pigment strength” of the colored transparent faceplate which is optimized for an emissive mode (nighttime) of operation and consequently the performance in a reflective mode (daytime) of operation is often far from acceptable.
There is another approach used today for single color signs and displays. A single colored light source may be used that matches the target color (e.g. red LEDs in stop lights and car tail lights). For large area color signs, architectural lighting and accent lighting it is common to have large sections of single colors using this method of dedicated color lights.
It is further known to construct signs, for example traffic signs, using an array of LEDs in which the LEDs are arranged in the form of the sign such as for example arrow symbols and “walk/stop” devices used in pedestrian crossings where the designed “native” emitted wavelength of light from the LED is the same as the viewed or perceived colored light of the viewer. Often such signs will further include a color filter or lens to give a more uniform color/intensity of emitted light or to shift the color (as in the case of the use of a white LED with an orange filter to generate an orange colored sign and/or display or lighting element).
White light emitting diodes (LEDs) are known in the art and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As is known white light generating LEDs (“white LEDs”) include a phosphor, that is a photo luminescent material, which absorbs a portion of the radiation emitted by the LED and re-emits radiation of a different color (wavelength). For example the LED emits blue light in the visible part of the spectrum and the phosphor re-emits yellow or a combination of green and red light, green and yellow or yellow and red light. The portion of the visible blue light emitted by the LED which is not absorbed by the phosphor mixes with the yellow light emitted to provide light which appears to the eye as being white.
It is predicted that white LEDs could potentially replace incandescent, fluorescent and neon light sources due to their long operating lifetimes, potentially many 100,000 of hours, and their high efficiency in terms of low power consumption. Recently high brightness white LEDs have been used to replace the conventional white fluorescent and neon lights in display backlight units. The colored materials with these white backlights come in a variety of forms such as vinyl films, colored polycarbonates and acrylics, color photographic transparency film, transparent colored inks for screen printing etc. All of these materials work on the same basic principle that they contain transparent colored dyes or pigments which absorb the unwanted colors of the backlight white and transmit the desired color to the viewer. Consequently they all function as color filters. Whilst the use of white LEDs has decreased the power consumption of backlit light emitting signs they still give a poor performance in terms of color saturation when operated in daylight conditions, often the color appears washed out.
U.S. Pat. No. 6,883,926 discloses an apparatus for display illumination which comprises a display surface which includes a phosphor material and at least one light emitting semiconductor device (LED) positioned to excite the phosphor by irradiating it with electromagnetic radiation of an appropriate wavelength. U.S. Pat. No. 6,883,926 teaches backlit and front lit variations. Such an apparatus finds particular application in vehicle instrumentation displays.
The present invention arose in an endeavor to provide an improved light emitting sign which provides greater flexibility and which in part at least overcomes the limitations of the known signs. Moreover it is an objective of the invention to provide a light emitting sign which offers increased brightness in emitted light with a reduced deterioration in color saturation and quality.